Systems and methods of coupling digitizing sensors to a structure

ABSTRACT

Systems and methods of coupling digitizing sensors to a structure are disclosed. A particular system includes a digitizing sensor node. The system further includes a bus including a plurality of conductive elements applied to a substrate. A first conductive element of the bus is coupled to the digitizing sensor node using a direct-write technique.

CLAIM OF PRIORITY

This application is a divisional of and claims priority from U.S. patentapplication Ser. No. 12/851,424, entitled “Method of Coupling DigitizingSensors to a Structure,” filed on Aug. 5, 2010, which claims priorityfrom U.S. Provisional Patent Application No. 61/232,685, entitled“Systems and Methods of Coupling Sensors to a Structure,” filed on Aug.10, 2009, each of which is incorporated herein by reference in itsentirety.

FIELD OF THE DISCLOSURE

The present disclosure is generally related to coupling sensors to astructure.

BACKGROUND

Many structures incorporate a distributed network of sensors in order tosuccessfully fulfill their function. For example, structural healthmonitoring (SHM) systems may use sensors distributed about a structure.SHM systems can provide the ability to detect and interpret adverse“changes” in a structure to reduce life-cycle costs and to improvereliability. SHM systems may utilize non-invasive detection sensors thatare integrated into a structure to continuously monitor components fordamage, such as cracks or de-lamination.

Implementing SHM can improve asset reliability, safety and readinesswhile reducing life-cycle costs. However, these improvements can come atthe expense of weight, power consumption and computational bandwidth.For example, an SHM system may utilize various sensors (such as straingauges, thermocouples and optical fibers) permanently mounted in regionsof interest. The number and location of these sensors may be limited dueto required infrastructure. The SHM system may have one or more analogcables (e.g., coaxial cables) from each sensing element (or sensor node)to a remote hardware location. These cables can be long, introducingelectromagnetic interference (EMI) susceptibility and signal attenuationdue to stray capacitance. Infrastructure of the SHM system, such as thecables, can add significant weight (e.g., more than about 6 kg/m̂2including connectors), and installation can be labor intensive.Furthermore, the cables and connectors introduce several failure points,as both the cables and the connectors are susceptible to environmentaland mechanical durability issues. Traditional data acquisition unitsalso can be bulky and expensive as hardware components are added toaccommodate the sensors. These concerns may be further exacerbated whenthe SHM system is operated in a harsh environment, such as extremetemperatures, shock, vibration and g-loading.

SUMMARY

Systems and methods of coupling digitizing sensors to a structure aredisclosed. A particular method includes applying one or morecommunication traces and one or more power traces to a structure usingat least one direct-write technique. The method may also includecoupling the one or more communication traces to at least one digitizingsensor. The method may also include coupling the one or more powertraces to the at least one digitizing sensor.

In a particular embodiment, a system includes a digitizing sensor node.The system may also include a bus including a plurality of conductiveelements applied to a substrate. A first conductive element of the busmay be coupled to the digitizing sensor node using a direct-writetechnique.

In a particular embodiment, a structure includes at least one structuralelement and at least one digitizing sensor coupled to the at least onestructural element. The system may also include a bus having a pluralityof conductive traces applied on the at least one structural elementusing a direct-write technique. One or more power traces of the bus mayprovide power to the at least one digitizing sensor. Additionally, oneor more communication traces of the bus may enable data communicationfrom the at least one digitizing sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an embodiment of an Intelli-Connector™digital structural health monitor (SHM) node;

FIG. 2 is a schematic of digital sensor node flexible circuit with tail;

FIG. 3 shows shorted traces due to melted Plasma Flame Spray (PFS)powder;

FIG. 4 shows meandering PFS traces on a carbon fiber reinforced plastic(CFRP) plate;

FIG. 5 shows a plate used to evaluate SHM compatibility;

FIG. 6 shows signal traces of a pitch-catch test with and without PFS;

FIG. 7 shows signal traces of a pulse-echo difference for PFS traces;

FIG. 8 shows signal traces of a pulse-echo with a magnet in a nearposition;

FIG. 9 shows signal traces of a pulse-echo with a magnet in a farposition;

FIG. 10 shows signal traces of a pulse-echo difference for JettedAtomized Deposition (JAD) traces;

FIG. 11 shows a schematic of a final integrated configuration;

FIG. 12 is a flow chart of a first embodiment of a method of fabricatinga bus;

FIG. 13 is a flow chart of a second embodiment of a method offabricating a bus;

FIG. 14 is a flow chart of a third embodiment of a method of fabricatinga bus

FIG. 15 shows a schematic of a test implementation; and

FIG. 16 shows a sample demonstration plate using a particularembodiment.

DETAILED DESCRIPTION

In some situations, analog data transfer may be undesirable, as it maybe susceptible to interference, may promote signal attenuation, and mayuse discrete wires for each sensor element. One technique that maymitigate the problems of analog data transfer is local sensordigitization. In a particular embodiment, a point-of-measurement (POM)data logging system may use a digital sensor infrastructure to co-locateanalog-to-digital chips with sensor elements so as to enable localdigitization thereby eliminating noise and attenuation. By convertinganalog signals into digital data at the point-of-measurement, theproblems of electromagnetic interference (EMI) and attenuation mayvirtually disappear. Local digitization can enable connecting sensorsserially, via a digital sensor-bus, to reduce total cable length. In aparticular embodiment, microprocessors may also be used to enable use ofa bus topology, thereby reducing cabling through use of a common serialdigital protocol. For example, data volume traversing the bus can alsobe greatly reduced by implementing signal processing and featureextraction techniques locally at the point of measurement. Programmablelogic may be used to differentiate data that needs to be reported to acentral processor from other information. Additionally, memory may beprovided locally at a sensor of a POM data logging system to log orbuffer sensed data.

The use of a point-of-measurement data logging system alone may notaddress all of the concerns of added weight, complexity and laborintensive installation of SHM systems, however, as there may still be acable harness to manually install. Durability of the cable harness mayalso still be of concern. In a particular embodiment, a cable-freedigital sensor-bus may be utilized to address these concerns. Forexample, the cable-free digital sensor-bus may be installed at astructure to be monitored using a direct-write (DW) technology. VariousDW technologies may enable conformal deposition of conductive and/ornon-conductive materials onto a variety of complex surfaces with a highdegree of precision. The DW technology may selectively depositelectrically conductive and/or insulative traces with very finetolerances either directly onto the structure or onto a layer that canbe applied to the structure. Such conformal DW traces may create astructural component with multifunctional capabilities, increasinginstrumentation reliability while greatly reducing weight as compared toa cable-based equivalent. For example, the cable-free digital sensor-busmay weigh less than about 25 grams per meter of length. Additionally,the cable-free digital sensor-bus may be flexible or semi-flexible toreduce installation concerns.

In a particular embodiment, a Plasma Flame Spray (PFS) DW technique maybe used. PFS enables a metal (e.g., copper) or a ceramic material to beelectrically liquefied for application on the structure. In anotherparticular embodiment, a Jetted Atomized Deposition (JAD) DW techniquemay be used. JAD enables a metal (e.g., silver) or a polymer (e.g., anultraviolet (UV) light curable epoxy) to be placed on the structure in amanner similar to an ink-jet printer and subsequently hardened.

For some retrofit applications, the structures to be monitored may betoo large to be shipped to a remote location for installation of acable-free digital sensor system. In a particular embodiment, suchstructures may be retrofit in situ. For example, DW techniques used forthese retrofit applications may be selected to avoid the use ofspecialized equipment or chambers that are not portable. To illustrate,a DW technique similar to a silk-screen method, described herein withreference to generation of a test system, may be used. In anotherexample, for a large-scale retro-fit embodiment, traces for thecable-free digital bus are applied to intermediary layers in aproduction environment and subsequently installed in situ on thestructure. For example, the traces may be patterned on a series ofsimple and modular intermediary layers, which may be adhered to thestructure on-site.

Using DW technology to pattern a cable-free digital bus may enablereliable transfer of electrical signals through patterned traces. In aparticular embodiment, the trace geometry, spacing and overallconfiguration of the cable-free digital bus may be precisely designedbased on design criteria of the system. In a particular illustrativeembodiment, sensor nodes of a SHM system may communicate via acontroller area network (CAN) via the cable-free digital bus. In such anetwork, four sets of traces and/or layers may be considered:communication, synchronization, power, and shielding. CAN protocolnetworks use a differential fixed-impedance protocol. Thus, twoconductors (CAN-high & CAN-low) with an impedance between around 100-130Ohms may be used for communications. The sensor nodes may optionally useindependent high-speed digital synchronization over an RS-422 protocol.This synchronization may also use a differential fixed-impedanceprotocol using two conductors (sync-high & sync-low) with impedancebetween about 100-130 Ohms. In a particular embodiment, for power, thesensor nodes have a maximum current draw of about 108 mA. TheIntelli-Connector™ HS version sensor node (available from Metis DesignCorporation (MDC)) may use a supply voltage of 28 VDC (power & ground)and a standby current draw of 30 mA. For a benchmark system (discussedwith reference to the tests that have been conducted), assuming thatthere would be one exciting node and no more than six sensing nodes at atime, the total current draw would be about 3.5 A. For this benchmarksystem, nominally six parallel traces may be used for communication,sync and power. In a particular embodiment, shielding may also beprovided. For example, parallel shield traces may separate thecommunication, sync and power traces. Also, since the pairs cannotreadily be twisted with DW techniques, a shield layer may be used aroundthe communication and sync traces to couple the pairs and protect themagainst EMI.

To resolve issues relating to continuous monitoring of aerospacecomponents, the MDC has developed an intelligent SHM infrastructurearchitecture called the Intelli-Connector™ digital sensorinfrastructure. The Intelli-Connector™ digital sensor infrastructure maybe used as a direct replacement of traditional instrumentation, such asoscilloscopes and function generators. The Intelli-Connector™ digitalsensors can reduce overall cable weight and increase signal fidelity bydigitizing at the point-of-measurement. Hardware requirements can alsobe reduced as compared to analog systems through distributed localprocessing. Sensor density can be reduced as compared to analog systemsby using a pulse-echo mode. In a particular configuration, multipledigital sensors can connect through a FireWire-type harness and can besynchronized for a pitch-catch mode. The Intelli-Connector™ digitalsensor infrastructure can facilitate remote SHM testing for multiplemethods including Lamb wave, Rayleigh wave or other guided waves (GW),modal analysis or frequency response (FR) and acoustic emission (AE). Ina particular embodiment, Lamb waves are analyzed to detect damage to asystem coupled to the Intelli-Connector™ digital sensor infrastructure.Lamb waves are a form of elastic perturbation that propagates in a solidmedium. In particular embodiments, Lamb wave analysis provides gooddamage size and detection range to sensor area ratio. Additionally, Lambwave analysis may provide good sensitivity and range may scale withinput power level (with some limitations). In an illustrativeembodiment, Lamb wave analysis can be used to detect and locate damageto a system based on how the damage slows down the waves. In anotherillustrative embodiment, Lamb wave analysis may detect and locate damagebased on how the damage reflects the waves. Further, Lamb wave analysiscan determine damage location using a single sensor node or usingmultiple sensor nodes cooperatively. For example, a single sensor nodecan be active to detect damage using a “pulse and listen” to initiate apulse and listen of a reflection of the pulse. In another example, afirst sensor node may be passive and a second sensor node may be active.The active sensor node may initiate a pulse and the passive sensor nodemay listen for the pulse to detect damage. The active sensor node andthe passive sensor node may be synchronized to enable determination oftiming information related to the wave.

While the benchmark system described below uses Lamb waves to detectdamage, as mentioned above, other techniques to detect damage could alsobe used. For example, to detect Lamb waves the Intelli-Connector™digital sensor infrastructure uses about 24V at the end of the bus tooperate. This may limit the number of sensors that can be served over afixed distance and spacing within the sensor-bus. In a particularembodiment, an acoustic emission (AE) system may require only about 3.3Vat the end of the bus to operate, which may reduce certain power lineconstraints. Additionally, synchronization may be important to implementLamb wave methods, thus sync lines may be used. However, using AEmethods may not require sync lines and may relax constraints onsynchronization of the communication lines as well, which may make thecommunication traces easier to design.

In various embodiments, a digitizing sensor node includes at least oneprocessor that processes information gathered by the digitizing sensornode and transmits the processed information via the bus. As an examplefor design of a cable-free digital sensor bus and for testing purposes,the digital sensor used was the Intelli-Connector™ digital sensor node,as shown in FIG. 1. The Intelli-Connector™ digital sensor node iscapable of local computation and processing. Designed mainly withpiezoelectric-based methods in mind that allow large-area coverage withreduced sensor densities, the Intelli-Connector™ digital sensor node canfacilitate remote SHM for guided waves (GW), frequency response (FR) andacoustic emission (AE) techniques. The Intelli-Connector™ digital sensornode is about 25 mm diameter by about 8 mm tall, weighs about 4 gramsand is encapsulated in urethane to conform to MIL-STD-810 and DO-160.The Intelli-Connector™ digital sensor node can excite arbitraryfunctions over 3.4 MS/s at 20 Vpp and can digitize 16-bit data at 1 MHzwith remotely programmable gains. In use, a SHM system may also include:connectors and cables to connect sensors to the SHM system; amplifiersto improve outgoing actuation or incoming signals; data acquisition(rate and resolution of sensor signal digitization); computation tocontrol and process data; memory to buffer or store data; communicationsto link sensors, hardware and users; and power to provide excitationsource and supply for electronics.

It is believed that in particular configurations, the Intelli-Connector™digital sensors infrastructure can withstand shock testing up to 90,000g's and has little impact on the structure being monitored. For example,the Intelli-Connector™ digital sensors infrastructure can use surfacemounted sensors. It is further believed that, in particularconfigurations, the Intelli-Connector™ digital sensors infrastructurecan withstand a reasonable range of MIL-STD temperatures (−40 F to 180F). The software-centric nature of the Intelli-Connector™ digitalsensors infrastructure may enable mass-produced hardware to be placedanywhere, rather than in custom shaped layers. Additionally, usingpiezoelectric-based methods, such as Lamb waves and Acoustic Emission ina pulse-echo mode, may enable large-area coverage with reduced sensordensities.

In a particular embodiment, a miniaturized version of anIntelli-Connector™ digital sensor may be used. The miniaturized versionmay have all packaging removed such that it is a wire-bond design. Byremoving the packaging, the weight of the sensor device may be reducedby 1 to 2 orders of magnitude. The form-factor and height may also bereduced by 50-80% in each dimension. The temperature survivability mayalso be improved since there will be no plastic cases or solder to melt.Further, the high-g, shock and vibration characteristics may be improvedas well based on the lower mass. Thus, the miniaturized version may beappropriate when weight, form-factor, temperature survivability, high-gshock or vibration characteristics are of concern.

As used herein, unless otherwise noted in the particular context, theterms sensor, digital sensor, digital sensor node, and sensor node referto any sensor used with a SHM system, such as the Intelli-Connector™digital sensor nodes described above. In a particular embodiment, thedigital sensors may be applied to a structure to be monitored using anadhesive. Adhesive thickness and stiffness may be selected to transmitthe most ultrasonic energy into the structure from the sensors and viceversa. For example, too thick of an adhesive material may provide alarge shear-lag effect, which may dampen out the wave within theadhesive layer. Too stiff of an adhesive may appear as a boundarycondition and create artificial reflections. In a particular embodiment,the adhesive layer acts as an impedance matched transition layer betweenthe sensor and the host structure. When a cable-free digital sensor busis used for data communications, thickness and properties of an adhesiveused to apply the bus may also be a concern. Beyond the shear coupling,another consideration when a cable-free digital sensor bus is used maybe heterogeneity of the traces with respect to the wave front. In aparticular embodiment, the layers beneath and directly surrounding anactuator or sensor are homogeneous so that there are no obstructions toexcited or sensed waves that could either create additional reflectionsor cause acceleration or deceleration of the signal, thus introducingalgorithm errors.

In a particular embodiment, a modified controller-area network (CAN)protocol is employed to daisy-chain multiple sensors on the same bus. Asused herein, unless otherwise noted in the particular context, a bus, aDW bus, and a cable-free digital bus refer to a set of conductiveelements generated using a direct write technique. In particularembodiments, such buses may be used to enable data communications, toprovide synchronization, to provide power, or any combination thereof,to one or more sensor nodes. Communications using the CAN protocol canbe sensitive to the transmission impedance present. Additionally, thedigitizing sensors (such as the Intelli-Connector™ digital sensors) mayfunction best with well conditioned power. Since impedance and powerconditioning can be related to bus characteristics, design of a DW bussystem for use with the digitizing sensors may be important. Forexample, DW trace widths and spacings may be selected to reduce theseconcerns. Additionally, shielding may be used in a DW bus system tofurther reduce these concerns. In addition, passive components may beselectively used in the DW bus system at the digitizing sensors, at hubsin front of CAN driver chips, or both to regulate and correct theimpedance to ensure reliable communication on a large network.

In a particular embodiment, coupling digitizing sensors with a DW bussystem may enable cable-free data transfer. Such embodiments may benefitfrom computational and power reducing benefits of point-of-measurementtechnology, as well as reduced mass and increased robustness offered byDW technology. Such an embodiment may be well suited for use inlarge-area composite structures, particularly for space applicationswhere mass may be important and operating conditions may include extremeenvironments.

Several factors may be considered to determine an appropriate method toattach the digitizing sensors and other hardware to the DW traces of thecable-free digital sensor bus. For example, one concern is robustnessand reliability of mechanical and electrical connections. Ease ofinstallation may also be of concern since the sensor network may berelatively large. Thus, complex procedures or procedures that require ahigh degree of precision may drive up the system cost substantially.Another concern may be obstruction of incident waves. Wave propagationmay be a consideration when routing traces in a flexible circuit andwhen routing traces of the DW bus.

Although the systems and methods disclosed focus on theIntelli-Connector™ digital sensors for KIM systems, in variousembodiments other types of sensors may be used in conjunction with acable-free digital sensor bus. For example, acoustic wave sensors, eddycurrent sensing devices, magnetic alloy devices, fiber optic devices,other sensors, or any combination thereof may be used. An actuatorseparate from the digitizing sensor node may be coupled to the bus,where the actuator and the digitizing sensor node are synchronized fordata gathering via a synchronization signal via the bus. To illustrate,in one embodiment, active acoustic emission technology may be used withthe disclosed cable-free digital sensor bus. Such active acousticemission technology may use embedded actuators and sensors within acomposite laminate to send a broadband pulse.

While various DW technologies may deposit traces on the structure in anypattern designed, reliably coupling digitizing sensors to a DW bus maybe problematic. For example, the digitizing sensors may be adapted touse a 6-pin FireWire cable for power and communication. The FireWirecable may include 6 wires. The digitizing sensors may use one pair ofwires for 5V power, one pair of wires for communicating digital data,and one pair of wires for high precision synchronization and remotefirmware re-programming. Typically, the 6-pins may be connected to adigitizing sensor via a soldered and overmolded wire at 6-leads on aprinted circuit board (PCB) of the digitizing sensor. Using a FireWireconnector to connect a sensor to the DW traces may present difficulties.For example, creating 3-D vias for the connectors using DW techniquesmay be difficult. Additionally, the DW traces may be very close togetherleading to space constraints. Further, a break in impedance on the mainsensor-bus trunk may cause communication reflections. Also, the FireWireconnectors add weight and potentially a failure point. In a particularembodiment, the sensors or a flexible circuit may be located directlyover the DW traces and bonded to make contact. However, this arrangementmay present alignment complexities.

As shown in the embodiment illustrated in FIG. 2, to couple a digitizingsensor 202 to a DW bus 204, traces of a flexible circuit tail 206 may berouted to a bottom (e.g., analog side) of the PCB that connects to apiezo/flexible circuit assembly of the digitizing sensor 202. Theflexible circuit tail 206 may be used to route digital traces tobreak-out locations where the digital traces can connect to the DWtraces of the DW bus 204. In a particular embodiment, the flexiblecircuit tail 206 is designed to shield the sensor signal from EMI and toproduce as little interference for the actuator for transferring aclean, omni-directional signal into the structure.

In a particular embodiment, during use of a digitizing sensor system,the FireWire cable may be connected to several devices in a network. Thenetwork may include a hub device. The hub may be coupled to the bus andbe adapted to receive signals from the digitizing sensor node via thebus using a first protocol and to convert the signals to a secondprotocol. For example, the hub may multiplex multiple cable harnesses,provide conditioned power to the DW bus 204, and convert CAN protocolsignals into USB protocol signals so that a simple PC can control thenetwork. When the FireWire cable is replaced with DW traces, flexibletraces may be used to attach to the DW traces to a PCB of the hub in amanner similar to that described above with reference to coupling adigitizing sensor to the DW bus 204.

In another embodiment, a flexible circuit 210 (such as one that isalready employed on the bottom of some SHM sensor nodes) may be used toconnect the sensor elements to a digital board, and to extend a flexiblecircuit “tail” (or “flex-tail”) outside the footprint of the sensor forthe DW sensor-bus attachment, seen in FIG. 2. The tail 206 may have atrace pattern similar to or identical to that of the DW bus(communication, sync, power & shield). Reasonable spacing may beprovided down a length of the tail. The flexible circuit tails 206 maybe perpendicular to the bus traces in orientation.

In a particular embodiment, DW techniques may be used to apply or towrite bus traces 212 to these flexible circuit tails (also referred toherein as “flex-tails”). To illustrate, the sensor may be coupled to thebus traces 212 by overwriting the flex-tail 206 with a direct-writeconductive material. For example, the flex-tail 206 may be bonded downto a structure 220. A Kapton overlay may selectively expose conductors214 at various points down the length of the flex-tail 206. Theflex-tail 206 may be bonded down to the structure 220 prior to a DWdeposition stage. The DW traces 212 may be patterned directly over theconductors 214 of the flex-tail 206. The DW traces 212 may then bereinforced with an epoxy insulator 222, which may also be part of the DWprocess. This arrangement may have a relatively small mass impact, andmay form a mechanically and electrically reliable and durableconnection.

In a particular embodiment where the structure 220 is a graphite fibercomposite (or another conductive material), to prevent the traces 212and 214 from shorting out on the conductive graphite fibers of thestructure 220, a non-conductive layer (not shown) may be placed on thestructure 220 before the traces are applied. For example, anapproximately 4 mil thick PEEK pressure sensitive adhesive (PSA) may beused for the non-conductive layer. Not only does this PSA provide anon-conductive layer, but it also provides a moisture barrier for thecomposite material, and may provide a simple way to remove the DW bus204 in the future if for some reason it needed to be repaired orreplaced.

An experiment was performed in order to determine whether the methodsdescribed above would be successful. Two copper-coated Kapton flexiblecircuits were bonded to either end of two composite plates. Parallelmetallic traces were patterned over the flexible circuits using DWmethods: PFS using copper, and JAD using silver. Positive results werenoted. In particular, nothing melted and the traces appeared to followthe layers without a problem. Additionally, there was good electricalcontinuity, and there appeared to be no mechanical problems. However,during the PFS process, metallic powder floated around and tended tostick where there was a gap, seam, or step on the composite plate(including superficial lines in the plate itself, at the interface withthe non-conducting film, and at the Kapton and copper steps). In someregions of the plate this powder could just be brushed away. However, atthe interface with the flexible circuit, near where the PFS headtravelled in order to write DW traces, this powder actually melted andformed a thin conductive path that could not be easily cleaned andshorted the parallel traces together, seen in FIG. 3. This is not aninsurmountable problem, but it should be taken into account whenapplying traces in this manner.

In a particular embodiment, the communication and sync traces shouldmatch a prescribed impedance to daisy-chain multiple sensors on a commonbus. Additionally, the power line traces should have enough area tocarry the specified current. Also, each of these traces should have lowenough resistance to allow the signals to be carried along the desiredtotal length for SHM coverage. Further, shield layers should be sized toprovide desired protection without interfering with bus functionally. Inparticular embodiments, two different DW techniques, PFS and JAD, areused to form a DW bus. PFS may use copper as a conductor and can patterntraces from about 0.3-1.5 mm wide (+50 μm) by about 0.05-1 mm thick (±10μm) with approximately a 2 mm pitch. JAD may use silver nano-particleink as a conductor and can pattern traces from about 0.1-0.3 mm wide (±5μm) by about 3-10 μm thick (+1 μm) with approximately a 0.1 mm pitch.Both PFS and JAD can be compatible with various spray-on epoxyinsulators.

In a particular embodiment, the PFS technique discussed above may beused to meet geometry constraints of the power line traces (alsoreferred to herein as “power traces” or “power lines”). Design of thepower traces according to a particular embodiment and experiments usedto validate these designs are discussed below.

Voltage carrying capacity may be proportional to trace pitch, andcurrent carrying capacity may be proportional to trace cross-sectionalarea. Two documents that provide guidance regarding design for currentand voltage are MIL-STD-275E, “Printed Wiring for Electronic Equipment”and IPC-2221 “Generic Standard on Printed Board Design,” both of whichare incorporated herein by reference in their entirety. From Table I in4A of MIL-STD-275E and Table 6-1 in the IPC-2221 standard, the powertrace spacing should be about 0.1 mm for up to 50 VDC assuming a Type A5assembly of external conductors with conformal coating. Similarly, fromFIG. 4A in MIL-STD-275E and FIG. 6-4 in the IPC-2221 standard, theminimum power trace cross sectional area to carry 3.5 A with a factor ofsafety of 2 would be about 0.16 mm̂2 assuming similar behavior toexternal etched copper conductors.

To validate these calculations for power traces applied using PFS, threesets of parallel copper traces with a 2.5 mm pitch were deposited on a75×75 cm square graphite/epoxy plate in a meandering pattern with atotal length of about 5 m, as seen in FIG. 4. The first set of tracesmeasured slightly below the minimum theoretical area at about 0.1 mm̂2,the second set of traces was just above the minimum area at about 0.2mm̂2, and the third set of traces was approximately twice the minimumarea at about 0.3 mm̂2. A 40 VDC, 15 ADC power supply was connected toeach pair of traces, and the current was measured with a multimeter. Allsets of traces successfully carried 40 VDC and 9.5 ADC for over 5minutes without any indication of failure.

There may be a limitation on total length of power traces for a givenresistivity, because as the resistance gets too high the voltage seen bythe last node in the sensor-bus may drop below a 24 V threshold expectedby the sensor nodes for excitation. To calculate the total allowablelength, the SHM system was treated as a large circuit diagram. The 100nodes were modeled as constant power dissipation sinks, with 93consuming 0.8 W while in standby, and 7 nodes consuming 3 W whilesensing. Each power trace between nodes was modeled as a small resistor.Then, setting the input voltage to 28 Vpp, Kirchhoff's laws could beused to solve for the maximum tolerable power trace resistivity. For thepreliminary design, a node spacing of ½ m was selected, as well ascopper traces with a conductivity equal to 58×106 S/m (IACS at 20° C.).Using these assumptions, the equations indicate a minimumcross-sectional area of 1 mm̂2

To determine the conductivity of the PFS traces, the resistance of thetraces and the geometry were measured from the plate described in FIG.4. The electrical conductivity is related to the trace resistance byR=L/(σ×A), where R is the resistance, L is the trace length, A is thetrace cross sectional area, and a the electrical conductivity. Usingthis equation, the measured electrical conductivity averaged 17×106 S/mfor the largest set of traces, 13×106 S/m for the middle set of tracesand 8×106 S/m for the smallest set of traces. These values are lowcompared to the copper standard, with differences likely due toimpurities, as well as geometry assumptions, since conductivity seems todecrease with increasing geometry. The consequence of this lowerconductivity value may be that: fewer nodes can exist on the bus asdesigned, the nodes may be spaced closer together, or the traces may bere-sized.

Various embodiments of cable-free digital sensor systems may usedifferent communications protocols, such as USB, Ethernet and FireWire.In a particular embodiment, a controller-area network (CAN) protocol maybe used for communications. The CAN protocol may be advantageous sincea) it is relatively high-speed, b) it has a serial architecture, and c)it has a forgiving network topology. For example, by allowing a range ofimpedance values from about 100-130 Ohms, along with good errorhandling, the CAN protocol may be less sensitive to inconsistencies andtolerances for fabricated traces than some other communicationprotocols. The CAN protocol is governed by ISO 11898, is a matureprotocol with more than 20 years of in-service applications, and isdesigned to allow devices to communicate without a host PC. It has amaximum transmission speed of 1 Mbit/sec for theoretically up to 2032devices over 1,000 m.

To design the communication traces, a Spice simulation model wasconstructed using measured values for copper and CAN controller elementsfor each of the SHM nodes in the network. A single high/low CAN pulsepair was input into the traces on one end, and the differential voltagewas displayed for the final node in the network. Impedance may be animportant parameter for digital communication. The impedance of a traceis a complex function of trace width, trace spacing, trace thickness,shield spacing, conductivity and dielectric constant for the insulationbetween the communication traces and the shield layers.

Initial iterations determined that the communication traces should beabout 0.25 mm by about 10 μm thick with approximately a 0.5 mm pitch.This is finer than PFS techniques are typically capable of depositing.PFS techniques may be used to make thick enough lines for power tracesbut may have positional tolerances that are not adequate forcommunication lines. However, the pitch capabilities of PFS may beimproved through the use of a 2D gantry or etching process to removematerial from a bulk trace (as opposed to an additive process). Also,hardware changes could be made that may reduce conductivity constraints.For example, a CAN repeater may be integrated within the sensor hardwareto boost the signal approximately every meter, which could reduceconductivity needs.

JAD techniques may be used accurately for controlled impedance, but onlyvery thin lines can be made, which may not be suitable to carry power.Accordingly, a hybrid approach may be used to form a cable-free digitalsensor bus. JAD may be used to apply the communication traces and PFSmay be used to apply power traces. A plate similar to the one shown inFIG. 4 was fabricated using JAD to measure the conductivity of thesilver nano-ink particles. While JAD was capable of achieving thespecified geometry of the communication traces, the measured resistancevalues of traces formed using JAD were 3 orders of magnitude higher thandesired for a 100 node system, approximately 1 kOhm per a 60 cm tracethat should be less than 1 Ohm. The increased resistance values appearto be related to the relatively thin traces characteristic of presentJAD techniques. Accordingly, while it is believed that JAD methods willenable greater thickness traces to be applied eventually, for testingpurposes a screen printing process was developed for silver epoxy usingchemical etched steel-shim templates. By using this screen printingprocess, thicker silver traces can be patterned than with the tested JADprocesses. Also, a finer pitch can be patterned than with current PFSmethods.

Based on testing, it was determined that the silver epoxy traces shouldbe at least 125 μm thick. The Spice model was updated to yield 0.33 mmwide traces with a 1.5 mm pitch. The synchronization traces may have anidentical geometry to the communication traces as they have the samecharacteristic impedance range using RS-422.

As discussed above, shield layers mainly serve to protect thecommunication traces and the sync traces from electrical and mechanicalinterference (EMI). However, digital signals may be less susceptible tonoise than analog signals, particularly in differential mode. Guidelinesindicate that even a minimal amount of metal, such as 1 μm layer, may bea suitable shield for digital signals against electrical interference upto 1 MHz. Magnetic interference may be harder to transmit and harder toprotect against. Using a 10 μm layer of metal may provide someprotection against magnetic interference up to 1 MHz; however, thislayer may either be significantly thicker or make use of a magneticfield absorbing material in order to provide better protection.

Beyond EMI, the shield layers may also play a role in CAN impedancecalculations. If in-plane and out-of-plane shield layers are placed farenough away from the communication traces, they may have littleinfluence. However, in a system designed for minimum overall weight andcross-sectional area, it may not be acceptable to space the layers farenough from the communication traces to reduce their influence. In thisarrangement, as long as the in-plane shield traces have a pitch equal toor greater than the communication traces, the in-plane shield traces mayhave negligible effect. The out-of-plane shield traces may play a majorrole in the impedance within reasonable geometry. Thus the out-of-planeshield traces may be designed to be about 0.5 mm above and below thecommunication traces. The remaining volume between the communicationtraces and sync traces, and the out-of-plane shield traces may be filledwith a dielectric material.

Another concern of electrical design of the bus may be selection of thedielectric material to electrically isolate all of the conductors. PFStechniques may be compatible with a variety of ceramic insulators thatoffer good dielectric values. However, stiffness of these ceramicinsulator materials may not be desirable. JAD techniques may becompatible with several UV-curable epoxies that provide good stiffness.However, there may be manufacturing difficulties with fabricating thickUV-cured sections. Accordingly, for testing purposes, a simple spray-onepoxy method was used to apply electrically insulating layers in acost-effective manner while still achieving the desired systemcharacteristics. A dielectric was selected that: a) had a dielectricconstant of about 3 between 10 kHz and 1 MHz (ASTM D-150), b) had aservice temperature of at least about 250° C. to survive a subsequentPFS process, c) had a room temperature cure cycle to reduce the effectof mismatched thermal expansion coefficients, and d) had a viscositysuitable for spraying. Multiple epoxies were procured that met thesecriteria at a range of viscosities for experimental validation of themanufacturing process and testing of the dielectric constant of eachmaterial. Actual dimensions of these insulating layers were selectedbased on the conducting layer designs.

There was some concern during design that the presence of the DW traceswould create guided wave (GW) scatter points, causing changes in signalphase and amplitude. To evaluate this concern, two carbon fiberreinforced polymer (CFRP) plates 500 measuring 75×75 cm by 2.5 mm thickwere instrumented with three pairs of sensor nodes 502 bonded onopposite ends of a pair of DW traces 506, as seen in FIG. 5. The firstplate used copper traces applied via PFS and representative of thedesigned power lines. The second plate used traces applied using JAD andrepresentative of the communication traces and sync traces. The sensornodes 502 included actuator and sensor pairs with a concentricallyplaced lead zirconium titanate (PZT)-5A washer and disc. The actuatorwas a ring with outer and inner diameters of 0.5 inches and 0.29 inches,respectively, while the sensor had a diameter of 0.25 inches. Both theactuator and the sensor had a thickness of 0.75 mm. Each sensor node 502can function both in the “pulse-echo” and “pitch-catch” modes. In the“pulse-echo” mode, the actuator is excited with a short time-span burstsignal and the sensor in the same pair is used to sense Lamb-wavereflections caused by the burst from damage zones near the sensor node502. In the “pitch-catch” mode, one or more sensors are used to recordthe response of the structure to excitation by an actuator at somedistance from the sensors. The sensor nodes 502 were connected to anAgilent 33220A function generator for the actuator, and the sensorsignal was recorded using an oscilloscope. Both pitch-catch andpulse-echo measurements were collected before and after the applicationof DW traces.

Baseline data for the first plate was collected over a range offrequencies (from about 50 to 500 kHz in steps of 50 kHz) before thetraces were deposited. Subsequently, data was collected for the samerange of frequencies after the PFS power traces were applied. Thesignals were filtered using a zero-phase, high-order Butterworth filterto eliminate high and low frequency noise outside the excited bandwidth.This was followed by a Hilbert transform to extract the pitch-catchsignal envelope.

A sample of the results is shown in FIG. 6 for a path between the twocentral sets of sensor nodes. As seen in FIG. 6, there is an observabledifference 606 in the pitch-catch signal transmitted across the powertraces 604 as compared to the pitch-catch signal transmitted without thepower traces 602. However, the difference is largely a result of someattenuation in signal amplitude rather than shape/phase. This isreflected in the values of the two metrics used to quantify this change,the signal amplitude metric and the signal correlation metric. Thesignal amplitude metric is indicative of the combined amplitude andphase change, whereas the signal correlation metric is insensitive tosignal amplitude changes. The average values of the two metrics acrossthe path between the central sensor node pairs at the tested frequencieswas tabulated. A larger drop in the amplitude metric (average of 26%) incomparison to the correlation metric (average of less than 2%) indicatedthat there was some attenuation of the signal transmitted across thepower traces, but no significant change in signal shape. Similar trendswere observed for the other pitch-catch paths.

The power traces also caused a small but detectable reflection inpulse-echo signal when the difference between the signals taken beforeand after the deposition of the power traces is examined, shown in FIG.7. Based on these reflections (which include both possible modes, i.e.,the zero-order symmetric (S0) and antisymmetric (A0) modes) the powertraces can be located relative to the nodes. For example, from thesignals in FIG. 7, and the wave speeds for the two modes at 50 kHzdetermined from the baseline pitch-catch signals, the location of thepower traces relative to the sensor node is 18.2 cm, which is 18 mm fromthe actual location. Thus, while there is some effect on the Lamb-wavesignals when compared before and after the deposition of the power linetraces, this may be largely a wave attenuative effect and should not beexpected to affect SHM performance.

The capability of the sensor nodes to detect damage with the powertraces deposited on the panel was also separately evaluated. Based onthe initial tests, a lower frequency of 30 kHz was chosen for the damagedetection tests. Multiple baseline pulse-echo signals were collected toexamine signal repeatability for the first plate (with the power traces)and to obtain threshold values. The signal repeatability was notadversely affected and threshold values were consistent with thoseobserved before the power traces were deposited. A cylindrical magnet12.7 mm in diameter by 6 mm tall was first placed half-way between asensor node and the power traces, and then half-way between the powertraces and a sensor on the oppose side.

During a first test, a clear reflection from the magnet is discernibleabove the threshold, as seen in FIG. 8. At this lower frequency, the A0mode reflection dominates much more strongly than the S0 modereflection, which is not discernible above the threshold. Based on theA0 mode reflection and using wave speeds estimated from pitch-catchtests, the magnet's location was estimated to be 9.7 cm, which is 2.5 mmfrom the actual position. Based on this result and the amplitude of thepulse-echo difference signal over the threshold seen in FIG. 8, a rangeof each node is estimated to be a circular region with radius ofapproximately 40 cm for this particular plate. Note that this estimateis independent of the presence of the power line trace, since the magnetis on the near side of the trace, relative to the sensor node.

Next, to evaluate whether damage can be detected if it is located beyondthe power traces, the magnet was placed in a second position. Signalsfrom this test are shown in FIG. 9, with a reflection from the magnetstill appearing above the threshold value. Based on the A0 modereflection, the magnet's location was estimated to be 31.2 cm, which isonly 5 mm from the actual position. Thus, the range estimate of thenodes from the earlier test is justified. This range is also consistentwith prior tests performed on similar CFRP panels using similar sensornodes.

While there was some effect on the Lamb-wave signal amplitude due to thepower traces when the signals before and after the DW process werecompared, the signal shape is not significantly affected. In particular,the ability for this method to detect and localize damage was notcompromised, as confirmed by the described test results using magnets tosimulate damage. In addition, the range of this method from each sensornode was not significantly changed by the presence of the power traces.

Once it was established that the power traces would not adversely affectdamage detection, there was confidence that the other traces would haveeven less of an effect since they were all significantly smaller withless deposited material. Nevertheless, for completeness a similar testmatrix was conducted for the second plate with the communication andsync traces applied using a JAD technique. Like the tests on the firstplate, a difference was observed in comparing results with and withoutthe silver communication and sync traces. In this case, however, theamplitude metric had a much smaller change (average of 8.9%) and thecorrelation metric had nearly no change (average of 0.6%). Similarchanges were observed for the pulse-echo results. From the pulse-echoresponse illustrated in FIG. 10, and the wave speeds for the two modesat 50 kHz determined from the baseline pitch-catch signals, the locationof the trace relative to the central sensor node was 1.4 cm, which is2.5 mm from the actual position.

One reason why neither sets of traces may have had a large effect on thewave propagation is due to a mitigation technique that was employed.From prior research it has been determined that placing a thin polymerlayer (about 125-250 μm) under potential wave scattering points candecouple these features from the wave propagation path. The lowstiffness of the thin polymer layer compared to the plate structure, theacoustic impedance mismatch, and the damping properties of the thinpolymer layer together may “hide” the stiffness of other features bondedabove the thin polymer layer as the wave travels underneath, so long asthe thin polymer layer is thick enough. In this case, a 125 μm thickPEEK pressure sensitive adhesive (PSA) was selectively applied to theplates prior to the DW processes. The PSA also prevented the traces fromshorting out on the conductive graphite fibers of the plates, andprovided a moisture barrier for the CFRP material. Furthermore, peelingthe PSA and the traces on the PSA off of the plates may provide a simpleway to remove the DW bus if for some reason it needed to be repaired orreplaced.

In a particular embodiment, one challenge with implementing a cable-freedigital sensor bus is integration of all of the system elements into acomplete design. A balance may be struck between maintaining theoptimized component characteristics and reducing overall bus geometryand mass. Additionally, consideration may be given to selectingappropriate manufacturing processes so that the designed configurationcan be fabricated. An embodiment of a final configuration can be seen inFIG. 11 (not to scale).

In the design illustrated in FIG. 11, each of the shield (S) traces1101-1103, the communication traces (“CAN”) 1104 (+/−) and the sync(+/−) traces 1105 may be about 330 μm wide by about 125 μm thick withapproximately a 1.5 mm pitch. Top 1106 and bottom 1107 shield layers ofabout 10 μm thick may surround all of these traces with approximately a1 mm pitch. An epoxy 1120 may fill in gaps between the traces 1101-1105and the shield layers 1106-1107 to maintain their relative placement andto provide electrical insulation. A power trace 1130 and a ground trace1131 may be placed on either side of a communication/sync stackincluding the communication traces 1104 and sync traces 1105. The powerand ground traces 1130-1131 may be about 1.25 mm wide by about 0.4 mmthick, with approximately a 12 mm effective pitch. An encapsulant 1140,such as a urethane, may be applied over the entire assembly. In thisarrangement, overall dimensions of the bus 1100 are approximately 1.35cm wide by approximately 1.25 mm thick, with a weight of around 25 g permeter of length. Thus, the entire weight of the benchmark systempreviously described would be about 2.5 kg for the cable-free digitalsensor-bus and about 0.5 kg for the sensors.

FIG. 12 illustrates a flow chart of a first particular embodiment of amethod of fabricating a bus. The method includes, at 1402, applying oneor more communication traces to the structure. At least onecommunication trace of the one or more communication traces may beformed using at least one direct-write technique. For example, the atleast one communication trace may be formed directly on the structureusing the at least one direct-write technique. In another example, theat least one direct-write technique may be used to deposit a conductivematerial to an intermediary layer, and the intermediary layer may beapplied to the structure.

In a particular embodiment, the one or more communication traces areformed in a plurality of layers separated by a dielectric material. Thedielectric material may be applied using the at least one direct-writetechnique. For example, at least two of the plurality of layers may belayers of a sprayed-on or painted-on conductor separated by thedielectric material. The at least one direct-write technique may includea deposition process, a spray process, a printing process, or anotherprocess. In a particular embodiment, two or more direct-write methodsare used to apply the one or more communication traces. In a particularembodiment, the one or more communication traces are embedded within asurface of the structure.

The method also includes, at 1404, coupling the one or morecommunication traces to at least one sensor. The at least one sensor mayinclude a first sensor and a second sensor. The first sensor and thesecond sensor may be of different types or of a same type. Toillustrate, a first sensor may include an actuator to generate awaveform used to test the structure, and a second sensor may include asensor to detect the waveform.

The method may also include, at 1406, coupling one or more indicators tothe one or more communication traces. The method may further include, at1408, monitoring the health or integrity of the structure using the atleast one sensor.

FIG. 13 illustrates a flow chart of a second particular embodiment of amethod of fabricating a bus. The method includes, at 1502, applying adielectric layer (such as, a pressure sensitive adhesive) to astructure. The dielectric layer may be applied to an area where the busis to be formed on the structure. The surface of the structure may betreated before the dielectric layer is applied. For example, the surfaceof the structure may be plasma treated, abraded, bead blasted, ortreated using another technique to improve adhesion of the dielectriclayer to the surface.

The method may also include, at 1504, applying a first conductive shieldlayer to the structure. For example, the first conductive shield layermay be applied over the dielectric layer. A first insulating layer maybe applied over the first conductive shield layer, at 1506.

The method may include, at 1508, applying one or more communicationtraces using at least one direct-write technique. The one or morecommunication traces may include at least one synchronization trace. Theat least one direct-write technique may include spraying or painting ona material that forms a conductor. In other examples, the at least onedirect-write technique may include a plasma flame spray technique,jetted atomized deposition technique, another printing or depositionprocess that applies a conductive material, or any combination thereof.The one or more communication traces may be coupled to one or morecommunication connectors of at least one digitizing sensor, at 1510. Ina particular embodiment, the at least one direct write technique is usedto form the one or more communication traces over the one or morecommunication connectors. For example, the one or more communicationconnectors may be applied (e.g., bonded or adhered) to the surface ofthe substrate, and the one or more communication traces may be writtendirectly over the one or more communication connectors.

In a particular embodiment, the one or more communication traces includeat least two communication traces, and the method includes, at 1512,applying one or more shield traces to the structure using the at leastone direct-write technique. The one or more shield traces are appliedbetween the at least two communication traces. In an illustrativeembodiment, the one or more communication traces and the one or moreshield traces may be formed substantially simultaneously. To illustrate,the at least one direct write technique may be used to form the one ormore communication traces and the one or more shield traces at the sametime. For example, a template or mask may be used to guide painting,spraying or screen printing of the one or more communication traces andthe one or more shield traces at the same time.

After the one or more communication traces are applied, the method mayinclude, at 1514, applying a second insulating layer over the one ormore communication traces. A second conductive shield layer may beapplied over the one or more communication traces, at 1516.

The method may include, at 1518, applying one or more power traces to astructure using at least one direct-write technique. In a particularembodiment, the one or more communication traces are applied using afirst direct-write technique and one or more power traces are appliedusing a second direct-write technique that is different than the firstdirect-write technique. The method may also include, at 1520, couplingthe one or more power traces to one or more power connectors of the atleast one digitizing sensor.

In a particular embodiment, the method may be performed remote from thestructure. For example, the one or more communication traces, the one ormore synchronization traces, the one or more shield traces, theconductive shield layers, the one or more power traces, the insulatinglayers, or any combination thereof may be formed on an intermediatelayer. Subsequently, the intermediate layer may be applied to thestructure. For example, the intermediate layer may include thedielectric layer formed on a temporary substrate, such as a releasepaper.

A particular embodiment of a fabrication method to form a cable-freedigital sensor system, such as the cable-free digital sensor busdescribed with reference to FIG. 11 or the benchmark system, isdescribed below. A series of multiple DW and spray operations may beused according to a particular embodiment.

The fabrication method is illustrated in FIG. 14 as a flow chart of athird particular embodiment of a method of fabricating a bus. The methodincludes, at 1602, applying a non-conductive layer, such as a polyetherether ketone (PEEK) pressure sensitive adhesive (PSA) in an area where abus will be laid. The method also includes, at 1604, applying a materialthat forms a conductive bottom shield layer. For example, a silver inkmay be sprayed over the PSA. In yet another example, the silver ink oranother material that forms a conductor may be painted or deposited onthe PSA. To illustrate, a printing technique, such as screen printing orGravure printing, may be used to apply the material to form theconductive bottom shield layer. In another example, the bottom shieldlayer may be pre-coated on the PSA.

The method also includes, at 1606, applying a dielectric material overthe bottom shield layer. For example, an epoxy material may be sprayedto a prescribed thickness over the bottom shield layer using a template.The epoxy material may be cured (e.g. using ultraviolet light). Themethod may also include, at 1608, applying a sensor system element tothe structure. For example, a sensor node may be bonded to the surface.In another example, a sensor node or another sensor system element maybe formed on the structure using a direct-write technique. A flexiblecircuit element, such as a flexi-tail, may be used to extend from thesensor system element into a bus direct-write region.

The method may also include, at 1610, applying a material that forms atleast one conductive trace. For example, a JAD technique may be used toapply the material to form the at least one conductive trace. In aparticular embodiment, a plurality of conductive traces may be formed ina pattern. The plurality of conductive traces may include communicationtraces, sync traces, in-plane shield traces, or any combination thereof.The at least one conductive traces may be applied at a locationapproximately centered in the bus direct-write region (i.e., an areawhere the bus is to be formed).

The method may also include, at 1612, applying a dielectric material toinsulate the at least one conductive trace and to build thickness for anext layer. For example, an epoxy material may be sprayed over the atleast one conductive trace. The epoxy material may be cured, e.g., usingultraviolet light.

The method may also include, at 1614, applying a conductive materialover the dielectric material to form a top shield. In a particularembodiment, the top shield may be formed using the same techniques usedto form the bottom shield. For example, a silver ink may be sprayed toform the top shield.

The method may also include, at 1616, applying a dielectric material(e.g., epoxy) over the top shield. A mask or template may be used tocontain a width of the dielectric material. The dielectric material maybe cured, e.g. using ultraviolet light.

The method may also include, at 1618, applying a conductor to form powerand ground traces. For example, the power and ground traces may beformed using a plasma flame spray technique. In a particular embodiment,the power and ground traces may be deposited on either side of thecommunication, sync and in-plane shield traces.

The method may also include, at 1620, applying a sealing material overthe entire assembly (i.e., the power and ground traces, communicationtraces, synchronization traces, in-plane shield traces, top shield,bottom shield and dielectric layers). A template may be used to achievea desired shape. The sealing material may include a urethane material oranother dielectric material.

The method may also include, at 1622, removing all masks and templatesused to form the bus. An area around the bus may also be cleaned (e.g.,to remove any copper dust or overspray of various materials).

For testing purposes, some deviations from the above specifiedmanufacturing process were used to accommodate available resources.Firstly, available sensor nodes were used to evaluate the digitalsensor-bus. The available sensor nodes used FireWire connectors forpower and CAN communications. The FireWire connector was not directlycompatible with the designed cable-free digital sensor bus. Therefore,an adapter was designed for the flex-tail of the sensor nodes, asillustrated in FIG. 15. The adapter enabled connection of the sensornode to the cable-free sensor bus by overwriting the adapter leads usingDW processes; however, rather than connecting the adapter directly tothe digital sensor node through a board connector, a small portion ofthe FireWire cable was crimped into a reciprocal board connector thatwas then plugged into the sensor node. Secondly, a silk-screen processwas used rather than a JAD process to apply the communication, sync andin-plane shield traces. The silk-screen process filled in achemical-etched steel shim with silver epoxy. Additionally, since thesilver traces were applied manually, only the communication lines werelaid down to demonstrate system functionality at lower risk. FIG. 15illustrates a schematic diagram of a particular embodiment as describedabove.

A test system was fabricated to test the designed digital sensor-bus andto demonstrate that all of the portions of the system integratedtogether. A mask was chemical-etched from steel shim stock to expose a“U” shaped channel on a 75×75×2.5 cm CFRP plate. The modifiedmanufacturing process described above was carried out, inserting fiveflex-tail adapters 1301-1305, as shown in FIG. 16. TheIntelli-Connector™ digital sensor nodes were bonded to the center of theplate in an isosceles triangle formation, and then plugged intoflex-tail adapters 1301-1305 in the middle of the sensor-bus fabricationarea. At the end of the bus, a small CAN terminator plug was inserted toestablish the correct line impedance for transmission. Finally, a hubboard was plugged into the front of the bus to convert CAN signals to aUSB protocol, which was then connected to a computer to control thedamage detection experiment. This arrangement replicated an experimentthat had previously been performed numerous times on a variety of typesof plates; however, in this case the cable-free digital sensor-bus wasused for power and communication in place of a traditional FireWirecable harness. A cylindrical magnet 12.7 mm diameter by 6 mm tall wasplaced on the plate using shear-gel in various locations within thetriangle formed by the sensor nodes. From the control computer,Lamb-wave pulse-echo tests were commanded for each of the sensor nodes,and their response was stored in comma-delimited files on the computer.

Subsequently, an analysis of the sensor data was performed using aMatlab™ algorithm to predict the location of the magnet based on thenearest intersection of the damage radii estimates from each of thesensor nodes. Following this procedure ten times with the magnet and tentimes without the magnet, no communication or power problems wereencountered and the presence of the magnet was correctly identified eachtime. Across these trials the average error in the distance predicted bythe algorithm was 7.5 mm.

The experiments indicate that using a cable-free digital sensor busformed using DW techniques for SHM applications may not reduce the SHMsensor nodes' ability to detect damage. Additionally, these experimentsdemonstrate suitable fabrication techniques that can be used to form acable-free digital sensor bus. Use of automated systems to prepare thecable-free digital sensor bus, such as video alignment tools to improvetrace placement accuracy, may further improve the systems and methodsdisclosed.

In a particular embodiment, elements of a sensor system other than thebus are also formed using direct-write techniques. For example, a sensorsystem may include a substrate, a bus having a plurality of conductiveelements that are formed on the substrate using a direct-writetechnique, and at least one sensor system element coupled to at leastone conductive element of the plurality of conductive elements, where atleast a portion of the at least one sensor system element was formed onthe substrate using the at least one direct-write technique. Thesubstrate may be the structure to be monitored or an intermediate layer.In a particular embodiment, the substrate is flexible and conforms to asurface of the structure to which the substrate is coupled.

The sensor system element may include a structural health sensor that isoperable to monitor structural health of the substrate or the structureto which the substrate is coupled. The structural health sensor maytransmit data related to the structural health of the substrate orstructure to another sensor system element via the bus. The sensorsystem element may include a signal processor, a multiplexor, anamplifier, a transistor or other circuit component, or any combinationthereof. For example, an ultrasonic sensor may be formed on thesubstrate or on a film applied to the substrate using one or moreprinting techniques, such as screen printing, flexography, or Gravureprinting.

The disclosed cable-free digital sensor systems and methods involvepatterning of communication, power and shield traces, or any combinationthereof, individually or as layers using one or more DW technologies.Sensor system elements may also be formed using the one or more DWtechnologies. The traces, the sensor system elements, or both, may beapplied directly to a structure or may be applied to an intermediarylayer that is subsequently applied to the structure to form a bus. Thebus may replace wires, cables and/or cable harnesses that wouldotherwise be used to connect sensors to a network at the structure. Thecable-free digital sensor systems may provide significant reduction ofweight, with a DW sensor-bus weighing approximately 20% of a comparabletraditional cable. Additional benefits may include increased reliabilityas a result of eliminating vibration and snag concerns, increaseddurability by eliminating fraying and corrosion, and reducedinstrumentation time and cost through an automated process. Inparticular embodiments, the cable-free digital sensor systems may beused to form large sensor arrays, such as those used to facilitatestructural health monitoring and/or health and usage monitoring.However, the cable-free digital sensor systems may be used in otherapplications as well. For example, the disclosed cable-free digital busmay be used with any type or combination of sensor elements that arecapable of outputting a digital signal. The sensors used may be of thesame type or different types. Additionally, the cable-free digital busmay support more than one network of sensors. For example, a structuralhealth monitoring system and a communication system, a distributedcomputing system, a flight control system another type of digitalsystem, or any combination thereof may operate simultaneously via thecable-free digital bus. In addition to digital sensors, the cable-freedigital bus may support communication between non-sensing elements, suchas computing elements (e.g., data processors), routing elements (e.g.,data routers or hubs), actuating elements (e.g., devices that arecapable of receiving a digital input and generating movement in responseto the digital input), indicators (e.g., instrument readouts, visualindicators, audible indicators), or any combination thereof.

The cable-free digital sensor systems may be particularly useful foraerospace applications (aircraft, unmanned aerial vehicles (UAVs),expendable & reusable spacecraft, satellites, space/lunar stations,missiles, etc). There may also be significant benefits from using thecable-free digital sensor systems in naval vessels (ships, subs,oil-platforms, unmanned underwater vehicles (UUVs), etc), groundvehicles (cars, trucks, tanks, unmanned ground vehicles (UGV), etc) andcivil infrastructure (buildings, bridges, roadways, pipelines,power-plants, wind-turbines, oil-rigs, towers, radomes, etc). Thecable-free digital sensor systems may also be useful forhuman/biological/biomedical health monitoring systems.

In-situ monitoring technologies that may use the disclosed cable-freedigital sensor systems have the potential for many economic benefits ina broad range of commercial and defense markets. There are severaladvantages to using cable-free digital sensor systems over traditionalpractices, such as reduced instrumentation time, fewer wires,elimination of component tear-down for installation, and thefacilitation of structural health monitoring (SHM) techniques. Thecable-free digital sensor systems can be utilized by structuresincluding aircraft, spacecraft, land-based vehicles, and sea vehicles.

In particular embodiments, SHM techniques enabled by use of thedisclosed cable-free digital sensor systems may have benefit such as areduced need for scheduled manual inspections. Additionally, such SHMmay enable the use of condition-based maintenance for efficientstructural design and need-based repair. Further, economic gain may bereaped from recaptured operational time and extension of component life.Also, a benefit may be realized in the case where a vehicle is savedfrom catastrophic failure.

An economic benefit of SHM enabled by the disclosed cable-free digitalsensor systems may be to enable the elimination of scheduled manualinspections in lieu of automated, in-situ, continuous inspection. Up to25% of the life cycle cost for both commercial and military aircraft iscurrently spent on inspection costs. For applications such as spacelaunch, these costs can exceed 80% of the life cycle expenditures. Thisis at least partially a consequence of labor intensive manualinspections. By implementing a continuous monitoring system, minimalhuman intervention may be required. For future reusable launch vehicledesigns, quick and accurate inspection enabled by the disclosedcable-free digital sensor systems may support rapid vehicle turnaround.

Additionally, benefit may be realized by embodiments in which thedisclosed cable-free digital sensor system is used for continuousmonitoring to guide maintenance. Financial savings in such embodimentsmay be due to the frequency of maintenance that as dictated by thedesign methodology used. For example, when a new vehicle is built, achoice of the design methodology may drive the inspection andmaintenance requirements of the components. Currently, many vehicles usea damage tolerant approach, which sets inspection intervals based upon apredicted critical flaw size to detect and repair a component prior tofailure. Continuous monitoring to guide maintenance may be referred toas condition-based maintenance (CBM). Several papers in the literaturehave claimed as much as a 25-33% decrease in total life costs by usingCBM over a traditional damage-tolerant design. Using CBM, a structure(such as an aircraft) may be continuously monitored, allowing thestructure to forgo regular inspection and maintenance intervals. CBMcombines many of the advantages of the safe-life design principles withthose of damage-tolerant design, in that the structure may be reliedupon in service for much longer using predictive models; however, thereare provisions for maintenance and repair when needed. Additionally, onestudy has shown that by using CBM to design structural components withlower factors of safety, the weight of an aircraft could be reduced byas much as 25%, thereby improving the range and fuel efficiency. Thereliability of the structure may be dependent on the accuracy andaccountability of the monitoring system. Thus, a dependable structuralhealth monitoring system may be used. These systems can be implementedon aircraft and other structures to replace regularly scheduledmaintenance overhauls and inspection cycles, and to only repair partswhen needed.

Further benefit may be achieved by harvesting additional operationaltime. For example, opportunity cost may be gained by being able tooperate a vehicle when it would have been otherwise detained forscheduled inspections or maintenance. For aircraft, many of theseinspections involve the tear-down of larger components and can take morethan a day before the aircraft is back in service.

For airlines that are attempting to reduce operational costs, SHMenabled by the disclosed cable-free digital sensor systems may provide asignificant competitive advantage, especially for a newly introducedaircraft. Current commercial aircraft may be designed for at least 20-25years of service and up to 90,000 flights (75,000 flights for 737's,20,000 flights for 747's, and 50,000 flights for 757 and 767's). Futurecommercial aircraft will likely require at least this endurance. It isbelieved that in recent years, the average major airline has spent about12% of its total operating expenses on maintenance and inspection,amounting to an industry total of about $9 billion a year. For smallerairlines this percentage may average about 20% a year, totaling aboutanother $1 billion in costs. Using these figures, implementing an SHMsystem with a condition-based maintenance philosophy has the potentialof saving the airline industry about $2.5-3 billion a year.

There are several documents issued by the Federal AviationAdministration (FAA) that regulate how aircraft may be inspected. TheFAR 25 lists the acceptable engineering design criteria for the damagetolerant design of an aircraft. The Code of Federal Regulations (CFR)Title 14 Part 145 requires that all maintenance be performed usingmethods prescribed by Advisory Circular (AC) 43.13-1B. Additionally, foreach certified commercial aircraft, an Aircraft Maintenance Manual (AMM)is created by the manufacturer in conjunction with the FAA CFR Title 14Part 39 that lists each component to be inspected, the inspectioninterval, the type of damage to be concerned about, and the suggestedmethods to be used for the inspection. While there is currently nospecific provision in any of the published directives for a structuralhealth monitoring system, it is believed that a structural healthmonitoring system could be implemented under the current regulationssince the structural health monitoring system would utilize similar orsuperior sensing methods, and the structural health monitoring systemwould monitor a vehicle more frequently.

Particular embodiments of the disclosed cable-free digital sensor systemmay be useful in vehicles with SHM systems. For example, in the Orionspacecraft there are three primary uses for damage detection systemsthat may use embodiments disclosed here. First, a cable-free digitalsensor system may provide detection and localization capabilities for abackshell of the Orion spacecraft to detect micrometeoroid/orbitingdebris (MMOC) damage. Next, the cable-free digital sensor system mayprovide leak detection and localization capabilities for the Orionspacecraft's crew module pressure vessel. Finally, a cable-free digitalsensor system may provide several unused flexible acquisition channelsin the existing hardware to support future expansion of other sensors,as well as modularity in the bus to accept further branches.

In another example, spacecraft and related systems may use thecable-free digital sensor systems disclosed. To illustrate, elements ofthe Constellation program, such as Ares V composite interstage andcomposite overwrapped pressure vessels (COPV), may use a cable-freedigital sensor system. The disclosed cable-free digital sensor systemsmay also be implemented as part of other crew exploration and reusablelaunch vehicles (CEV and RLV) for quick turn-around times, expendablelaunch systems for pre-launch go/no-go decisions, and in theinternational space station to detect impacts or other damage.

The disclosed cable-free digital sensor systems may also be used incommercial markets for SHM. For example, aging fixed and rotary-wingaircraft may be retro-fit to include a cable-free digital sensor system.Likewise, cable-free digital sensor systems may be installed in newfixed and rotary-wing aircraft. Unmanned vehicles (UAV and UCAV) andcommercial aircraft may also use the disclosed cable-free digital sensorsystems. Additionally, cable-free digital sensor systems may bebeneficial for use in water craft (e.g., ships, submarines, carriers),land-based vehicles (e.g., cars, trucks, tanks) and civil infrastructure(e.g., bridges, tunnels, buildings).

Although the present specification describes components and functionsthat may be implemented in particular embodiments with reference toparticular standards and protocols, the disclosed embodiments are notlimited to such standards and protocols. For example, standards forInternet and other network transmission (e.g., TCP/IP, UDP/IP, HTML,HTTP, CAN) represent examples of the state of the art. Such standardsare periodically superseded by faster or more efficient equivalentshaving essentially the same functions. Accordingly, replacementstandards and protocols having the same or similar functions as thosedisclosed herein are considered equivalents thereof.

The illustrations of the embodiments described herein are intended toprovide a general understanding of the structure of the variousembodiments. The illustrations are not intended to serve as a completedescription of all of the elements and features of apparatus and systemsthat utilize the structures or methods described herein. Many otherembodiments may be apparent to those of skill in the art upon reviewingthe disclosure. Other embodiments may be utilized and derived from thedisclosure, such that structural and logical substitutions and changesmay be made without departing from the scope of the disclosure.Additionally, the illustrations are merely representational and may notbe drawn to scale. Certain proportions within the illustrations may beexaggerated, while other proportions may be reduced. Accordingly, thedisclosure and the figures are to be regarded as illustrative ratherthan restrictive.

Although specific embodiments have been illustrated and describedherein, it should be appreciated that any subsequent arrangementdesigned to achieve the same or similar purpose may be substituted forthe specific embodiments shown. This disclosure is intended to cover anyand all subsequent adaptations or variations of various embodiments.Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the description.

The Abstract of the Disclosure is provided with the understanding thatit will not be used to interpret or limit the scope or meaning of theclaims. In addition, in the foregoing Detailed Description, variousfeatures may be grouped together or described in a single embodiment forthe purpose of streamlining the disclosure. This disclosure is not to beinterpreted as reflecting an intention that the claimed embodimentsrequire more features than are expressly recited in each claim. Rather,as the following claims reflect, claimed subject matter may be directedto less than all of the features of any of the disclosed embodiments.Thus, the following claims are incorporated into the DetailedDescription, with each claim standing on its own as defining separatelyclaimed subject matter.

The above-disclosed subject matter is to be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments, which fall withinthe scope of the present disclosure. Thus, to the maximum extent allowedby law, the scope of the present disclosure is to be determined by thebroadest permissible interpretation of the following claims and theirequivalents, and shall not be restricted or limited by the foregoingdetailed description.

What is claimed is:
 1. A system, comprising: a digitizing sensor node;and a bus comprising a plurality of conductive elements applied to asubstrate, wherein a first conductive element of the bus is coupled tothe digitizing sensor node using a direct-write technique.
 2. The systemof claim 1, further comprising a hub coupled to the bus, wherein the hubis adapted to receive signals from the digitizing sensor node via thebus using a first protocol and to convert the signals to a secondprotocol.
 3. The system of claim 1, wherein the digitizing sensor nodecomprises at least one processor that processes information gathered bythe digitizing sensor node and transmits the processed information viathe bus.
 4. The system of claim 1, further comprising an actuatorseparate from the digitizing sensor node and coupled to the bus, whereinthe actuator and the digitizing sensor node are synchronized for datagathering via a synchronization signal transmitted via the bus.
 5. Astructure, comprising: at least one structural element; at least onedigitizing sensor coupled to the at least one structural element; and abus comprising a plurality of conductive traces applied on the at leastone structural element using a direct-write technique, wherein one ormore power traces of the bus provide power to the at least onedigitizing sensor, and wherein one or more communication traces of thebus enable data communication from the at least one digitizing sensor.6. The structure of claim 5, wherein the at least one structural elementcomprises a conductive material, and wherein a dielectric material isapplied between the at least one structural element and the bus.
 7. Thestructure of claim 5, wherein the at least one digitizing sensor isadapted to gather data regarding structural integrity of the at leastone structural element, and to transmit the data as a digital signal viathe one or more communication traces to a structural health monitoringsystem.
 8. The structure of claim 5, further comprising an actuatorcoupled to the at least one structural element.
 9. The structure ofclaim 8, wherein one or more synchronization traces of the bus enablesynchronization between the actuator and the digitizing sensor.
 10. Thestructure of claim 5, wherein the one or more communication tracesinclude at least two communication traces, and wherein the structurefurther comprises one or more shield traces between the at least twocommunication traces.
 11. The structure of claim 5, further comprising afirst conductive shield layer applied to the structural element, whereinthe one or more communication traces are applied over the firstconductive shield layer.
 12. The structure of claim 11, furthercomprising a second conductive shield layer over the one or morecommunication traces.
 13. The structure of claim 12, further comprising:a first insulating layer between the first conductive shield layer andthe one or more communication traces; and a second insulating layerbetween the one or more communication traces and the second conductiveshield layer.
 14. The structure of claim 5, further comprising adielectric layer applied to the structure, wherein the one or morecommunication traces are applied over the dielectric layer.
 15. Thestructure of claim 14, wherein the dielectric layer comprises a pressuresensitive adhesive, wherein the dielectric layer is coupled to thestructure via the pressure sensitive adhesive.
 16. A method comprising:receiving structural health information at a digitizing sensor coupledto at least one structural element; and converting the structural healthinformation to digital data; transmitting a signal corresponding to thedigital data to a central processor via one or more communication tracesof a bus comprising a plurality of conductive elements applied to thestructural element using a direct-write technique.
 17. The method ofclaim 16, wherein the central processor corresponds to a structuralhealth monitoring system.
 18. The method of claim 16, further comprisingdifferentiating between first data to report and second data, whereinthe digital data includes the first data.
 19. The method of claim 16,further comprising receiving an acoustic signal from an actuator via theat least one structural element, wherein the structural healthinformation is determined based on the acoustic signal.
 20. The methodof claim 19, wherein the actuator is coupled to the digitizing sensorvia a synchronization trace of the one or more communication traces.